1. Field of the Invention
The present invention generally relates to an ultrasonic receiving apparatus. More specifically, the present invention is directed to an optical converting type ultrasonic receiving apparatus capable of detecting ultrasonic waves by using light.
2. Description of a Related Art
Conventionally, when three-dimensional images of an object to be inspected are acquired by employing ultrasonic waves (beams), a plurality of two-dimensional images as to sectional views along depth directions thereof have been acquired, and then the acquired two-dimensional images have been synthesized with each other. This two-dimensional image is acquired in such a manner that an object to be inspected is scanned by employing a one-dimensional sensor array equipped with a position sensor. Furthermore, a plurality of two-dimensional images acquired in a time sequential manner are synthesized with each other, so that a three-dimensional image of this object can be obtained.
However, in accordance with this image acquisition method, there is a time lag along the scanning direction of the one-dimensional sensor array. As a result, sectional images acquired at different time instants are synthesized with each other, so that a synthesized image would become burring. As a consequence, this three-dimensional image acquisition method is not suitable for imaging an object having motion such as a living object or a biological body.
In order to acquire a three-dimensional image in real time, while such a two-dimensional sensor array is necessary required by which a two-dimensional image of an object can be acquired by employing a sensor array without scanning the object, development of such a sensor array is strongly desired.
Generally speaking, in ultrasonic diagnosing apparatus, as elements used to transmit and receive ultrasonic waves (namely, transducer elements or ultrasonic probes), piezoelectric ceramics typically known as PZT (lead titanate zirconate), and also polymer piezoelectric elements such as PVDF (polyvinyl difluoride) have been employed. While these elements are employed, such a method of manufacturing the above-described two-dimensional array has been considered. However, in the case where the above-explained ultrasonic transmission/reception piezoelectric elements such as PZT and PVDF are employed, theses piezoelectric elements are necessarily required to be processed in very fine modes, and furthermore, wiring works for a very large number of very fine piezoelectric elements are also required. Therefore, it is practically difficult to manufacture these piezoelectric elements in very finer modes and also in higher integration degrees, as compared with those of the presently-available piezoelectric elements. Also, even if these technical difficulties may be solved in near future, then other problems will apparently occur. That is, crosstalk among these piezoelectric elements will be increased, S/N ratios will be deteriorated due to increases of electric impedance caused by very fine wiring lines, and/or electrode portions of very fine piezoelectric elements will be readily destroyed. As a consequence, such a two-dimensional sensor array with employment of the above-described PZT and PVDF can be hardly realized.
On the other hand, as ultrasonic sensors without using such piezoelectric materials such as PZT, another detecting type of sensor (will be referred to an “optical detecting type” hereinafter) is known by which ultrasonic signals are converted into optical signals, while utilizing optical fibers. As such optical detecting type ultrasonic sensors, the following sensors have been reported, namely, ultrasonic sensor with employment of a fiber Bragg grating (will be abbreviated as an “FBG” hereinafter, see “Underwater Acoustic Sensor with Fiber Bragg Grating” written by TAKAHASHI et. al. of National Defense Academy in Japan, OPTICAL REVIEW Vol. 4, No. 6, in 1997. pages 691 to 694), and an ultrasonic sensor with employment of a Fabry-Pérot resonator (will be abbreviated as an “FPR” hereinafter) structure (see “Fabrication and Performance of a Fiber Optic Micro-Probe for Megahertz Ultrasonic Field Measurements” written by UNO et. al., of Tokyo Institute of Technology, T. IEE Japan, Vol. 118-E, No. 11, in 1998).
The above-mentioned document written by TAKAHASHI et. al. clearly describes such a fact that when the fiber Bragg grating is employed as the ultrasonic sensor, this ultrasonic sensor could sense ultrasonic waves in such a relatively low frequency range defined on the order of 20 kHz at a certain sensitivity. However, this document never describes such ultrasonic waves in megahertz frequency ranges which are used in actual ultrasonic diagnosing operations. As a consequence, in order that such an ultrasonic sensor is actually available in ultrasonic diagnosing operations, ultrasonic operations with respect to such ultrasonic waves in frequency ranges higher than that of the ultrasonic sensor written as the example in this document should be confirmed. Also, if required, various conditions for achieving better sensitivities in such higher frequency ranges should also be acquired.